A large majority of electronic circuits require one or more power supplies for operation. Typically, these power supplies must be stable. However, even though the power supplies are considered stable, the power supply levels generated for some applications usually vary during operation. For instance, a 3.3 volt power supply may generate anywhere from 2.8 volts to 3.7 volts during its operation and still be considered (i.e., rated) a 3.3 volt power supply. Similarly, a 5 volt power supply may generate anywhere from 4.2 to 5.6 volts during its operation and still be considered (rated) a 5 volt power supply. These varying type of power supplies are employed in powering electronic circuitry on computer chips.
Today, certain computer chips are capable of being supplied with two different power supply levels. For instance, a chip may be supplied with either a 5 volt or a 3.3 volt power supply. Furthermore, certain chips in the market today utilize different power supply levels for different portions of the chip. In other words, a portion of the chip may be powered by a power supply at one level (e.g., 5 volt supply), while another portion of the chip is powered by a power supply at a different level (e.g., 3.3 volt supply). Also a portion of the chip may be powered by a power supply at one level (e.g., 3.3 volt supply), while another portion of the chip is capable of being supplied with more than one power supply level (e.g., 3.3 volts or 5 volts).
Some of the circuitry on these dual or multi-power supply level chips are sensitive to the power supply level under which they operate. For instance, circuitry powered at one level may not operate in the same manner when powered at another level. For example, if an input buffer were powered by multiple power supply levels, its trip points would be effected by the change in the level, such that the trip point would be different for the different power supply levels. These circuitries must be able to compensate for the differences in the power supply levels. Also these circuitries would also need an indication of the power supply level. For example, trip-point selection circuitry would be needed to set the trip point for the input buffers according to whether the chip is being powered by one power supply level (i.e., 3.3 volts) or another power supply level (i.e., 5 volts). In these instances, the circuits must be programmed according to the power supply level which is being utilized.
In the prior art, these power supply level dependent circuits can be programmed in one of two ways. First, a set of registers located on the chip itself is available for the user to program the level of the power supply currently being used. Depending on how the user programs the register, the circuitry is able to compensate for the various power supplies levels which can be received. However, the use of registers on-chip requires the use of memory storage on-chip. Memory storage is limited on-chip and generally there is a need to limit the use of on-chip memory. Furthermore, the use of these on-chip registers requires the time of a system designer to program them.
Second, external pins may be used to indicate to the chip the level of the power supply being used. For instance, when a single pin is used to indicate the power supply level for a chip, the chip is capable of operating at two different power supply levels. When the pin is in one state (e.g., a logical 0), the power supply level is at a first level, while if the pin is in the other state (e.g., a logical 1), then the power supply is at a second level. If multiple portions of a chip are capable of operating at different power levels, then each separate portion would require a separate pin. The number of pins on a chip directly affects the size of the chip. The more pins a chip has, the larger it is. To minimize the chip size, the number of pins must be reduced. Thus, there is a need to allow a chip, or portion thereof, to be powered by more than one power supply level, while keeping the pin count down.
In the prior art, especially with respect to power level sensitive input buffer circuitry, the basic input/output system (BIOS) is used to determine the chip power supply. Where a chip is capable of operating at a two power supply levels (e.g., a 3.3 or 5 volt power supply), the BIOS writer must write two versions of the BIOS, one for operating with one power supply level (e.g., 3 volt) and one for operating with the other power supply level (e.g., 5 volt). It is desirable to have the voltage supply level automatically sensed at power-up for the voltage sensitive circuitry, such that no work is required from the BIOS writer. Furthermore, the trip-points of input buffers are not properly set until approximately 50 prefetch cycles have been completed and after the input to the chip has been used for internal operation. Moreover, in the prior art, as the input trip-point is not set properly until a certain number of prefetch cycles, the wrong trip-point may be used, causing the chip to hang and never recover. Also, if the BIOS programs the trip-point improperly, the chip may not work as expected. It is desirable to start sensing power immediately after power-up and also to select the trip-point of the input buffer before any input is sampled. This almost guarantees that the chip will function properly.
As will be shown, the power supply level detector of the present invention eliminates the need to have on-chip programmable registers to indicate to the chip the power supply level. Also the power supply level detector of the present invention eliminates the need to use external pins to program circuitry requiring knowledge of the power supply level to operate correctly. The power supply level detector of the present invention senses the peripheral power supply of the I/O buffers to determine what power supply is applied to the chip during the power-up reset sequence. Once the power supply of the I/O buffers is determined, the present invention sets the trip point of the buffers automatically. Furthermore, after setting the trip-point, the present invention shuts itself off to save power.